To protect and defend military platforms, such as ships, aircraft, and ground-based installations, it is known to provide countermeasure systems that detect incoming threats such as enemy aircraft or missiles. Known systems detect incoming threats and then deploy defensive countermeasures in an attempt to divert or destroy the threat. These systems are referred to as open-loop systems since no immediate determination as to the type of threat or the effectiveness of the countermeasure is inherent in their operation. Due to the inefficiency of open-loop systems, closed-loop systems have been developed.
There are known performance benefits to using a directional, laser-based, closed-loop infrared countermeasure system to defeat infrared guided missiles. In a closed-loop system, the incoming missile type is identified, and the countermeasure system generates a jam signal according to the specific characteristics of the incoming missile. This optimized jam signal is directed at the missile and induces it to execute a turn-away maneuver from its intended target. An additional feature of closed-loop techniques is their ability to monitor the classification and identification processes during the jamming operation, so as to provide a direct measure of the countermeasure effectiveness as well as an indication of necessary corrective actions in the generation of the jam signal. It will be appreciated that the benefits of the closed-loop system performance must be balanced against the cost of upgrading existing infrared directional countermeasure systems to employ a closed-loop capability, or against the cost of developing an entirely new closed-loop system.
One possible configuration for introducing a closed-loop receiver into an open loop-directional countermeasure system is to use a high-resolution tracking sensor in parallel with an infrared detector assembly. Accordingly, an independent receive channel, which is a separate optical path, must be added to the detection system along with a separate expensive cryogenically cooled detector. The cost and size impact of such a configuration to the countermeasure system is often prohibitive.
Another approach is to incorporate an infrared detector assembly into the countermeasure system and optically divert (or split-off) a portion of the receive optical signal for the high resolution tracking sensor. Unfortunately, this approach causes at least a 50% loss of receive signal strength for both the track sensor and the receiver, and it entails the cost for adding a cryogenically cooled detector. Another problem with this approach is that the optical apertures required by the sensor and the detector may require a larger overall assembly to accommodate them.
Based upon the foregoing, a need arose in the art for a single imaging infrared receiver having a focal plane array capable of frame rates sufficient to provide sensor data for three primary closed-loop countermeasure functions: a passive high-resolution tracking capability, the ability to receive and process laser signals, and finally, the ability to perform countermeasure effectiveness measurements. Further, the receiver function must not be impaired by the transmission of the laser jam signal.
The foregoing problems have been addressed in U.S. Pat. Nos. 6,369,885 and 6,674,520, both of which are incorporated herein by reference. With the implementation of the advantageous features of the aforementioned patents, additional needs have become apparent. The prior art tracking devices utilize pointers, which are considered to be expensive, and of a size sufficient to inflict undesirable drag penalties on aircraft that employ them. Further, many previous pointers utilized in closed-loop infrared countermeasure (IRCM) systems are unable to efficiently and accurately track incoming threats due to slow responses and limitations in the pointer apparatus. For example, some known pointers cannot track through the nadir position, because their gimbal geometry requires unachievable acceleration of the pointer assembly about the azimuth axis to do so. Dual path pointers, wherein the laser transmit and receiver paths are maintained separately, require precision alignment of the pointer optics to maintain parallelism; this alignment is difficult to achieve, and if it is not maintained, the device fails to operate as intended. In order to overcome the aforementioned problems it is known to provide a two-axis agile mirror for fine tracking carried by a two-axis coarse gimbal. However, such an assembly is found to be quite expensive. The prior art pointers are also lacking inasmuch as a significant amount of laser back scatter into the receive path is encountered, and, in addition, the previous systems require precision difficult to achieve and maintain alignment of pointer transmit and receive path mirrors. Previous dual path pointers also require laser transmit path holes in the receive path fold mirror located between the pointer and the camera. Accordingly, such a construction results in an undesirable amount of signal loss due to blockage of the camera receive signal path.
Based upon the foregoing, it is apparent that there is a need in the art for an improved tracking device, which offers the ability for the camera and countermeasure laser to employ the same optical path. There is also a need for a simplified gimbal construction, which allows for tracking of an object about and through nadir, and, which uses lower cost components to achieve the desired performance.